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2-16 WIRELESS/RADIO POINTS - 1 Electromagnetic radiation--as we learned from Maxwell's Equations--happens for all alternating currents in wires. All metallic transmission media are antennas! Efficiently built and installed antennas transmit radio signals when built to be an appreciable fraction of the operating wavelength or larger. Propagation is characteristic of the wavelength due to terrestrial factors and possible antenna construction at various sizes. Radio signals are made and used practically as low at KHz frequencies (imagine the wavelength and antenna sizes needed) up to 100 GHz or where practical transmitting and receiving electronics is not available.

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2-17 WIRELESS/RADIO POINTS - 2 Various modulation techniques are used depending on application. Impairments depend on wavelength, modulation system employed, and atmospheric absorption, multipath distortion and other reflection caused problems, and even refraction causing phenomena. Co-channel and adjacent channel interference can limit cause problems in channelized systems. Unlicensed (but legal) services are increasingly greatly, and spread spectrum techniques (especially in unlicensed bands) are exploding in use. Full characterization is not known.

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2-19 Lower frequencies/longer wavelengths require large antennas, propagate via ionosphere reflections (e.g. worldwide short wave radio); higher frequencies such as microwave have highly directional antennas, require "line of sight" transmission, and are used for point to point and satellite transmission.

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2-21 This ring of allowable locations (i.e. 23,000 miles above the equator) for geosynchronous satellites is known as, "The Clarke Belt", in honor of science fiction author Sir Arthur C. Clarke, who is said to have devised the concept. Clarke is now in his 80s and lives in Sri Lanka, the only non-native with permanent residency status there. He is ailing but still actively publishing- -his new book just came out--and participates in a weekly web cast. He also hosts a science program on public television.

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2-23 GEOSYNCHRONOUS SATELLITE COVERAGE & BANDWIDTH Coverage for each satellite can be for about 1/3 to 1/4 the Earth's surface from each satellite, but footprints can be constrained much smaller by directional antenna techniques as well as longitude at which a given satellite is assigned. Very northern and southern locations are problematical but possible with large ground antennas mounted high enough to have a distant horizon to "see" the satellite. Each satellite has a number of 6 MHz channels, each called a "transponder", that relays ground transmissions received on one frequency called an "uplink" to a paired "downlink" frequency. A number of frequency bands are allocated internationally for geosynchronous satellites but frequency reuse limit satellite spacing to 1 degree at best. There are a lot of geosynchronous satellites in the Clarke Belt, mostly filling up all the usable longitudes.

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2-24 Geosynchronous Orbit Geosynchronous (adj.): geo-, earth and synchronous, going on at the same rate and exactly together. A satellite in geosynchronous orbit circles the earth once each day. The time it takes for a satellite to orbit the earth is called its period. For a satellite's orbit period to be one day, it must be approximately 35,786 kilometers (19,323 nautical miles or 22,241 statute miles) above the earth's surface. That is a lot higher than the Shuttle ever goes (usually about 300 kilometers). To stay over the same spot on earth, a geostationary satellite also has to be directly above the equator.